When producing paper pulp a digestion liquor called black kraft liquor is obtained, which liquor besides numerous solubilized organic materials also contains the chemicals used for digesting the wooden raw material, i e sodium hydroxide and sodium sulphide, now mainly in the form of sodium carbonate and sodium sulphate. The black kraft liquor is generally concentrated before combustion for recovery of the energy content of the organic matter and for recovery of the cooking chemicals as so called green liquor.
Different processes have been proposed for recovering the energy and cooking chemicals, whereby the Tomlinson process is one, using a huge combustion furnace with steam raising heat recovery. Another proposed process is the so called cyclone combustion process, in which pyrolysis and combustion of the organic material takes place in a cyclone chamber. This latter process has, however, not gained any commercial success so far, and has only been tested in pilot plant.